1. Field of the Invention
The present invention relates to a welding flux and, more particularly, to a welding flux for stainless steel.
2. Description of the Related Art
Conventionally, stainless steel parts are joined together by arc welding including tungsten inert gas welding (TIG), metal inert gas welding (MIG), submerged arc welding (SAW), and flux cored arc welding (FCAW). In tungsten inert gas welding, a tungsten bar serves as an electrode, and an arc generated between the electrode and the stainless steel parts serves as a heatsource for welding. During welding, inert gas is supplied from an arc welding torch to the portions of the stainless steel parts to be joined to avoid oxidization at the electrode, the welding pool, the arc, and the adjacent heated area, such that the welding portion can smoothly harden and cool to form a weld pass. However, problems including insufficient penetration depth, non-uniform penetration depth, and/or formation of a wild and shallow welding pool often occur when using the tungsten insert gas welding to form a weld pass with complete joint penetration in the stainless parts. This is due to a minor change in the alloy element in the welding pool. Thus, it is an issue to increase the penetration depth in conventional tungsten inert gas welding for increasing yield and assuring complete joint penetration.
FIGS. 1A, 1B, and 1C show pre-processing of two stainless steel parts for enhancing conventional tungsten inert gas welding and the cross sectional view of the stainless steel parts after welding. To eliminate the problem of formation of a wide, shallow welding pool, a side 11 of each stainless steel part 1 is milled by a cutter 2 to form a bevel face 12. A groove is formed at the butt joint of the bevel faces 12 of the stainless steel parts 1 for carrying out tungsten inert gas welding by using a welding rod 100 and an arc welding torch 3 forming the tungsten bar electrode. A weld pass 13 is formed after welding. However, formation of the bevel faces 12 increases the penetration depth of the weld pass 13 at the cost of increased processing difficulties, increased manufacturing costs, and longer processing time. Furthermore, the weld pass 13 has a poor welding structure, poor joining strength, and an uneven top surface.
U.S. Patent Publication No. 2005/0199317 (Taiwan Patent Publication No. I231239) discloses a welding flux for use in arc-welding of stainless steel parts and a method of welding stainless steel parts using the welding flux. The welding flux consists essentially of over 70 wt % of manganese peroxide (the base material) and less than 30 wt % of at least one activator selected from a material group that includes zinc oxide, silicon dioxide, chromium oxide, titanium oxide, molybdenum dioxide, and iron oxide.
With reference to FIGS. 2A and 2B, when joining two stainless steel parts 1 using arc welding, a welding flux 4 containing the base material and the activator mentioned above is mixed in a liquid carrier to form a paste-like flux, and a thin layer of the paste-like flux is coated over the joint of the stainless steel parts 1 by a brush 40 to allow subsequent welding of the stainless steel parts 1 using an arc welding torch. A weld pass 13 is formed after arc welding. Since welding spatters are scarcely generated near the weld pass 13, the surface of the weld pass 13 is almost flush with the unmolten surfaces of the stainless steel parts 1. A wide, narrow, complete joint penetration is obtained at the weld pass 13 and can be seen from the cross sectioned sample of the weld pass 13.
With reference to FIGS. 3A and 3B, the welding quality is enhanced by adding the base material (manganese peroxide) and the activator. Thus, the welding flux 4 effectively improves the gradient change in the surface tension of the liquid, molten metal in the welding pool 10, which affects the flow of the liquid, molten metal in the welding pool 10. Specifically, the gradient change in the surface tension of the liquid, molten metal depends on the temperature coefficient of the surface tension of the welding pool 10 while the temperature coefficient depends on the presence of active elements.
With reference to FIG. 3A, in a case that there is no active element in the welding pool 10 (or the active element in the welding pool is less active), the surface tension of the welding pool 10 will decrease when the temperature of the arc generated by the arc welding torch 3 increases. The surface of the liquid, molten metal flows outward from a center of the welding pool 10 (“the exterior surface tension flow”). The weld pass 13 thus formed is wide and shallow. On the other hand, when active elements are present in the welding pool 10, the surface tension of the welding pool 10 will increase when the temperature of the arc increases. The surface of the liquid, molten metal flows inward towards the center of the welding pool 10 (“the interior surface tension flow”). The weld pass 13 thus formed is narrow and deep.
Although the conventional welding flux 4 contains activators (the active elements), most of the welding flux 4 is manganese peroxide (the base material) that is of little help to activation. Furthermore, coating of the welding flux 4 on top of the sides 11 of the stainless steel parts 1 is troublesome and complicates the welding procedure. Further, the particle size of the powder welding flux 4 is large and, thus, difficult to coat, resulting in uneven thickness when the welding flux 4 is coated to the stainless steel parts 1 and leading to uneven penetration depth of the resultant welding pass.
Taiwan Patent Publication No. I297629 discloses a welding flux for stainless steel containing activators including titanium oxide (25-40 wt %), chromium oxide (25-30 wt %), silicon dioxide (10-30 wt %), molybdenum sulfide (10-30 wt %), and molybdenum oxide (5-15 wt %) to increase the penetration depth.
However, generation of welding spatters that are difficult to remove is liable to occur due to the high molybdenum sulfide content in the welding flux disclosed in Taiwan Patent Publication No. I297629, resulting in an uneven surface of the weld pass and in difficulties in cleaning. Furthermore, the costs of the welding wax are increased by some of the activators (such as chromium oxide) that are expensive and have a high percentage in the welding flux. Furthermore, the particle size of the powder welding flux is large and, thus, results in uneven thickness when the welding flux is coated to the stainless steel parts, leading to poor penetration, insufficient penetration depth, and an insufficient depth/width ratio of the weld pass easily causing deformation of the stainless steel parts. Thus, a need exists for an improved welding flux for stainless steel to enhance the welding quality of stainless steel parts.